Composite panels having improved fluid impermeability

ABSTRACT

A prepreg composite material that includes a fiber layer and a resin comprising a thermoset resin component, a curing agent and a fibrous micropulp. The micropulp component is an aramid fiber having a volume average length of from 0.01 to 100 micrometers. The prepreg is useful in composite panel construction for minimizing fluid permeation into the cured structure. This prepreg is particularly suitable for making honeycomb sandwich panels. Film adhesives, liquid and paste resins containing aramid fiber micropulp are also disclosed.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates generally to structural panels made fromcomposite materials and, in particular, to honeycomb sandwich panels.

2. Description of Related Art

Composite materials are used extensively in the aerospace industry andin other situations where high strength, stiffness and light weight aredesired. Monolithic composite structures typically include fibers andpolymer resin as the two principal elements. A wide range of fiber typeshas been used in composites. Glass, graphite, carbon and aramid fiberare common. The fibers can be chopped, randomly oriented, unidirectionalin orientation or woven into fabric. The resin matrix is usually athermoset material that includes resin, curing agents and otherperformance modifiers. A fibrous reinforcement (sheet, tape, tow, fabricor mat) pre-impregnated with resin is known as a prepreg.

Core structures are used for certain applications. Honeycomb is apopular core material for such panels because of its high strength toweight ratio and resistance to fatigue failures. Honeycomb cores aremade from a wide variety of materials with aramid paper and aluminumbeing the most common. Prepreg facesheets are bonded to each side of thecore.

Both monolithic and core composite structures have to be capable ofmaintaining an adequate mechanical performance after exposure to wet andmoist environments. Honeycomb sandwich panels are particularly prone tomoisture ingression into the cells of the core during service.

Technical reports on this subject include “Moisture Ingression inHoneycomb Core Sandwich Panels” by Cise et. al. in the Journal ofMaterials Engineering and Performance, Volume 6(6), 732, December 1997;“Caution, Honeycomb Core Can Be Dangerous To Your Program's Health” byCampbell in Corrosion Reviews, 25 (1-2), 13-26, 2007 and Loken et. al.in “Water Ingression Resistant Thin Faced Honeycomb Cored CompositeSystems with Facesheets Reinforced with Kevlar® Aramid Fiber and Kevlarwith Carbon Fibers” published by E.I. Du Pont de Nemours, Wilmington,Del.

SUMMARY OF THE INVENTION

In one embodiment, the present invention is directed to a prepreg for acomposite structure having a resin component comprising a thermosetresin, at least one curing agent and a micropulp of high tenacity fiber,said high tenacity fiber having a volume average length of from 0.01 to100 micrometers.

In one embodiment, this invention is also directed to the use of such aprepreg to function as a fluid impermeable barrier in a cured compositematerial panel.

In one embodiment, this invention also relates to alternative film,liquid or paste resin materials incorporating micropulp that serve toreduce fluid permeation in composite structures

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A and 1B are representations of views of a hexagonal shapedhoneycomb.

FIG. 2 is a representation of another view of a hexagonal cell shapedhoneycomb.

FIG. 3 is an illustration of honeycomb provided with facesheets.

FIG. 4 is another illustration of honeycomb provided with facesheets.

FIG. 5 is an illustration of an assembly for a monolithic compositestructure.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1A is a plan view illustration of a honeycomb 1 of this inventionand shows cells 2 formed by cell walls 3. FIG. 1B is an elevation viewof the honeycomb shown in FIG. 1A and shows the two exterior faces, orsurfaces 4 formed at both ends of the cell walls. The core also hasedges 5. FIG. 2 is a three-dimensional view of the honeycomb. Shown ishoneycomb 1 having hexagonal cells 2 and cell walls 3. The “T” dimensionor the thickness of the honeycomb is shown in FIG. 2. Hexagonal cellsare shown; however, other geometric arrangements are possible withsquare, over-expanded and flex-core cells being among the most commonpossible arrangements. Such cell types are well known in the art andreference can be made to Honeycomb Technology by T. Bitzer (Chapman &Hall, publishers, 1997) for additional information on possible geometriccell types. FIG. 3 shows a structural sandwich panel 5 assembled from ahoneycomb core 6 with facesheets 7 and 8, attached to the two exteriorfaces of the core. The facesheet material is a prepreg. In somecircumstances, an adhesive film 9 is also used. This adhesive film islocated such that one surface of the film, the inner film surface, islocated against the core surface with the other surface, the outer filmsurface, in contact with the first prepreg facesheet. Normally, thereare between two and twenty-five prepreg facesheets on either side of thecore. FIG. 4 shows an alternative embodiment of a sandwich panel 10assembled from a honeycomb core 11 with adhesive film 12 attached to thetwo exterior faces of the core and prepreg facesheets 13 located on topof the film. On one side of the assembly, the exterior side of thestructure is a surfacing film 14. FIG. 5 shows an embodiment of anassembly of prepreg sheets 15 on a tool 16 to make a monolithiccomposite structure.

A necessary component of the invention is a prepreg. The prepreg resinincludes a thermosetting resin, or a combination of thermosetting resinssuch as phenolic, epoxy, cyanate ester, bismaleimide, and the like,curing agents and a sufficient amount of a fibrous micropulp to providea prepreg that, on cure, will function adequately as a fluid barrier ina cured panel. This prepreg is referred to as a fluid or moisturebarrier prepreg. The term prepreg includes material forms such asmolding compounds which are strands of chopped fiber impregnated withchopped resin and towpreg which is a continuous filament fiber towpre-impregnated with resin.

The thermosetting resins which are combined with the fiber layer to formthe moisture barrier prepreg in accordance with the present inventionmay be selected from a phenolic, epoxy, cyanate ester, polyimide orbismaleimide resin. Exemplary epoxy and cyanate ester resins includeglycidylamine type epoxy resins, such as triglycidyl-p-aminophenol,tetraglycidyldiaminodiphenyl-methane; glycidyl ether type epoxy resins,such as bisphenol A type epoxy resins, bisphenol F type epoxy resins,bisphenol S type epoxy resins, phenol novolak type epoxy resins, cresolnovolak type epoxy resins and resorcinol type epoxy resins; cyanateesters, such as 1,1′-bis(4-cyanatophenyl) ethane (e.g. AroCy L-10,available from Huntsman, Inc., The Woodlands, Tex.), 1,3-Bis(4-cyanateophenyl-1-1-(1-m-ethylethyl idene) benzene (e.g. RTX366,available from Huntsman). Epoxy resins are preferred. Especiallypreferred epoxy blends include a mixture of trifunctional epoxy and adifunctional bis-F epoxy.

The epoxy resin may be composed of trifunctional epoxy, difunctionalepoxy and a wide variety of combinations of trifunctional anddifunctional epoxies. Tetrafunctional epoxies may also be used.Exemplary trifunctional epoxy include triglycidyl p-aminophenol andN,N-Diglycidyl-4-glycidyloxyaniline (e.g. MY-0510 or MY-0500 availablefrom Huntsman). Exemplary difunctional epoxies which may be used in theresin include Bis-F epoxies, such as GY-281, LY-9703 and GY-285 whichare also available from Huntsman). Bis-A epoxies, such as GY-6010(Huntsman) and DER 331 (Dow Chemical, Midland, Mich.) are suitableBisphenol-A type epoxies and may also be used. An exemplarytetrafunctional epoxy is tetraglycidyl diaminodiphenyl methane (e.g.MY-721, MY-720 and MY-9512 available from Huntsman). Other suitableepoxies include phenol novolac type epoxy, cresol novolac epoxy andresorcinol type epoxy. Preferred bis-F epoxies include GY281 and GY285that are available from Huntsman.

The curing agent is preferably an amine, an anhydride, a substitutedurea or dicyandiamide. Combinations of curing agents can also be used.Exemplary curative agents include dicyandiamide,3,3-diaminodiphenylsulfone (3,3-DDS), amino or glycidyl-silanes such as3-amino propyltriethoxysilane, CuAcAc/Nonylphenol (1/0.1),4,4′-diaminodiphenylsulfone (4,4′-DDS),4,4′-methylenebis(2-isopropyl-6-m-ethylaniline), e.g., Lonzacure M-MIPA(Lonza Corporation, Fair Lawn, N.J.),4,4′-methylenebis(2,6-diisopropylaniline), e.g., Lonzacure M-DIPA (LonzaCorp.). Combinations of curatives such as 3,3-DDS and dicyandiamide mayalso be used.

The fibrous micropulp component of the prepreg resin is generally from0.05 to 10.0, more preferably from 0.05 to 6.0 and most preferably from0.05 to 3.0 weight percent of the fully formulated resin composition andhas an average surface area ranging from 25 to 500 square meters pergram, preferably ranging from 25 to 200 square meters per gram and morepreferably ranging from 30 to 80 square meters per gram. The volumeweighted average length ranges from 0.01 micrometer to 100 micrometers,preferably ranging from 0.1 micrometer to 50 micrometers and morepreferably from ranging from 0.1 micrometer to 10 micrometers. Thismicropulp is a fibrous organic material that includes an intermeshedcombination of two or more webbed, dendritic, branched, mushroomed orfibril structures. The fiber feedstock for the micropulp has a tenacityof at least 3 grams per denier (2.7 grams per dtex).

Fibers suitable for conversion into micropulp used in the presentinvention can be made from polymers of aliphatic polyamides, polyesters,polyacrylonitriles, polyvinyl alcohols, polyolefins, polyvinylchlorides, polyvinylidene chlorides, polyurethanes, polyfluorocarbons,polybenzimidazoles, polyphenylenetriazoles, polyphenylene sulfides,polyoxadiazoles, polyimides, aromatic polyamides, or a mixture thereof.More preferred polymers are made from aromatic polyamides,polybenzoxadiazole, polybenzimidazole, or a mixture thereof. Still morepreferred organic fibers are aromatic polyamides ((p-phenyleneterephthalamide), poly(m-phenylene isophthalamide), or a mixturethereof). More particularly, the aromatic polyamide fibers disclosed inU.S. Pat. Nos. 3,869,430; 3,869,429; 3,767,756; and 2,999,788 arepreferred. Such aromatic polyamide organic fibers and various forms ofthese fibers are available from E. I. du Pont de Nemours and Company,Wilmington, Del. (DuPont), under the trademarks Kevlar® and Nomex®.

As employed herein the term aramid means a polyamide wherein at least85% of the amide (—CONH—) linkages are attached directly to two aromaticrings. Additives can be used with the aramid. In fact, it has been foundthat up to as much as 10 percent, by weight, of other polymeric materialcan be blended with the aramid or that copolymers can be used having asmuch as 10 percent of other diamine substituted for the diamine of thearamid or as much as 10 percent of other diacid chloride substituted forthe diacid chloride of the aramid. Para aramid fibers and various formsof these fibers are available from DuPont under the trademark Kevlar®and from Teijin, Ltd., under the trademark Twaron®.

Other suitable commercial polymer fibers for conversion to micropulp areZylon® PBO-AS (Poly(p-phenylene-2,6-benzobisoxazole) fiber, Zylon®PBO-HM (Poly(p-phenylene-2,6-benzobisoxazole)) fiber, Dyneema® SK60 andSK71 ultra high strength polyethylene fiber, all supplied by Toyobo,Japan; Celanese Vectran® HS pulp, EFT 1063-178, supplied by EngineeringFibers Technology, Shelton, Conn.; CFF Fibrillated Acrylic Fibersupplied by Sterling Fibers Inc, Pace, Fla; and Tiara Aramid KY-400SPulp supplied by Daicel Chemical Industries, Ltd, 1 Teppo-Cho, SakaiCity Japan. Natural fibers, such as cellulose, cotton and wool fiberscan also be utilized.

Micropulp suitable for use in this invention can be made by the methoddisclosed in United States Patent Publication No. 20030114641 A1 toKelly et al. The micropulp can be incorporated into the resin mix in oneof two ways. It may be added directly into the resin or as a resinand/or solvent pre-mix. Suitable solvents include methanol,methylethylketone and dichloromethane. Methods of preparing suchmicropulp premixes are described in US Patent Publication No.2004/0191192 A1 to Blankenbeckler et al. Resins incorporating from 0.05to 10.0, more preferably from 0.05 to 6.0 and most preferably from 0.05to 3.0 weight percent of micropulp prepared in this way can also be usedfor resin transfer molding (RTM), pultrusion and filament windingapplications. Such processes, preferably utilizing epoxy resins, arewell understood by those skilled in the art.

Other performance enhancing materials or modifiers may also beincorporated into the resin formulation. Non-limiting examples of these,which may be used alone or in combination, include diluents, viscositycontrol agents, flame-retardants, tougheners, UV stabilizers andanti-fungal agents. Viscosity control additives are frequently used. Theviscosity control agent is preferably a thermoplastic material thatdissolves in the thermosetting resin.

Exemplary viscosity control agents include thermoplastic polyetherimidessuch as ULTEM® 1000P which is available from General Electric(Pittsfield, Mass.); micronized polyethersulfone (PES) such as 5003P,which is available from Sumitomo Chemical Co., Ltd. (Osaka, Japan) andpolyimide MATRIMID® 9725, which is available from Huntsman. ULTEM® 1000Pand micronized PES are preferred. Micronized PES is especiallypreferred. The amount and type of viscosity control agent that is addedto the epoxy resin mixture may be varied to provide the desiredviscosity.

Further information on typical formulations is available in the chapteron epoxy resins in the ASM Handbook, Volume 21, Composites, 2001.

An exemplary prepreg resin formulation for moisture barrier prepreg isas follows:

-   1 to 70 parts by weight of an epoxy;-   5 to 40 parts by weight of an amine curing agent;-   1 to 30 parts by weight of a viscosity control agent; and-   0.1 to 10 parts by weight of fibrous micropulp.

In another embodiment, the prepreg resin formulation for moisturebarrier prepreg is as follows:

-   10 to 40 parts by weight of a trifunctional epoxy resin;-   10 to 40 parts by weight of a difunctional epoxy resin;-   11 to 25 parts by weight of an aromatic curing agent;-   0 to 3 parts by weight of a non-aromatic curing agent; and-   5 to 15 parts by weight of a viscosity control agent-   0.1 to 10 parts by weight of fibrous micropulp.

The finished resin is applied to the desired fibers to form a prepreg.The resin content of the prepreg may be varied depending upon a numberof different parameters in order to achieve desired mechanical andstructural properties for the composite panel. It is preferred that theprepreg has a resin content from 30 to 45 weight percent and a fibercontent of from 55 to 70 weight percent. Methods of combining the resinand fiber to make prepreg, either by a solvent or solvent-free (hotmelt) method, are well known to those skilled in the art and are furtherdetailed on page 56 of “Manufacturing Processes for Advanced Composites”by F. C. Campbell, Elsevier and in “Prepreg Technology” productliterature from Hexcel Corporation.

The reinforcement fibers that are used in the prepreg can be any of thefiber materials that are used to form composite laminates. Exemplaryfiber materials include glass, aramid, carbon, ceramic and hybridsthereof. The fibers may be woven, unidirectional or in the form ofrandom fiber mat. Preferred woven forms are plain, satin or twill weavefabrics woven from carbon or glass fibers. Such materials are availablefrom Hexcel or BGF Industries Inc., Greensboro, N.C.

In another embodiment of this invention, a further necessary componentis a honeycomb. Exemplary honeycomb materials for use in this inventioninclude aluminum, aramid, carbon or glass fiber. Preferred materials aremeta-aramid such as Nomex®, para-aramid such as Kevlar® and 5052 or 5056grade aluminum alloys. The dimensions of the honeycomb can be variedwidely. Typically, the honeycomb cores will have ⅛ to ½ inch (3.2-12.7mm) cells with the cores being ¼ inch (6.4 mm) to 2 inches (50.8 mm)thick. Cell size is the diameter of an inscribed circle within the cellof a honeycomb core. The final core densities are normally in the rangeof 2-12 lb. per cu.ft. (32-192 kg/m³). Honeycomb core is available fromcompanies such as Euro-Composites, Elkwood, Va., Hexcel Corporation,Casa Grande, Ariz. and M. C. Gill Corporation, El Monte, Calif.

In accordance with the present invention, fluid barrier prepreg, 15 inFIG. 5, may be assembled on a mold, 16 in FIG. 5, and cured to form acomposite material structure having an improved resistance to fluidpermeation. The composite structure may further comprise a honeycombcore. Depending on the desired design of the structural part the numberof prepreg sheets or plies required can be between one and thirty.Although the prepreg of this invention can provide structuralproperties, the composite panels may also contain prepreg sheets that donot possess fluid impermeation properties but function purely in astructural role. Such prepregs are widely available in the compositematerials industry, an example being Cycom® 970 from Cytec EngineeredMaterials, Tempe, Ariz. The relative amounts of different prepregs usedcan be optimized to meet design needs.

The preparation of monolithic or core based structural composite panelsfrom prepreg and core involves techniques well known in the art. Theprincipal methods are autoclave, hot press and vacuum bag curingtechnology. These techniques are explained in more detail in the chapteron curing in the ASM Handbook, Volume 21, Composites, 2001. The amountof vacuum, pressure and heat required to cure and bond the panelcomponents may be varied depending upon the particular resin formulationand the amount of resin in the prepreg. In general, sufficient pressuremust be applied to the prepreg to ensure that the resin adequately flowsthroughout the structure. In the case of a honeycomb panel structure,resin flow to provide adequate fillet formation on the core edges isalso important.

In another embodiment of this invention, the aramid fiber micropulp maybe incorporated into a film adhesive that forms part of the sandwichpanel assembly. Although such film adhesives are normally located nextto the core as shown at 9 in FIG. 3 or as an outer surfacing layer asshown at 14 in FIG. 4, they could also be positioned between prepregsheets. These adhesives are based on epoxy, bismaleimide, phenolic,polyimide and cyanate ester chemistries similar to those in prepregresins. Aluminum, silica and flame-retardants are also commonly usedingredients. Films can also be provided with a carrier cloth, an examplebeing a knit tricot cloth of polyester yarn. The amount of aramid fibermicropulp in a film formulation, excluding the carrier, is generallyfrom 0.05 to 10.0, more preferably from 0.05 to 6.0 and most preferablyfrom 0.05 to 3.0 weight percent of the fully formulated filmcomposition. The pulp has the same properties as that used in theprepreg resin. Both film and prepreg may be utilized in the sameassembly as fluid impermeable facesheets.

Yet another embodiment of this invention is the incorporation of aramidfiber micropulp into an adhesive paste or syntactic for use ininsert-potting or core edge filling applications. Insert potting is thebonding of location attachments into the sandwich panel positioned wherefasteners must be put through the panel. This bonding is normally doneas a co-curing operation with the prepreg curing. Core edge filling iscarried out after panel cure and seals the core edges, 5 in FIG. 1B,from damage and moisture ingress. The process involves mechanicallyremoving the honeycomb core ½ to 1 cell deep about the entire peripheryof the panel edge and then hand filling the resulting open core areawith resin. After the resin cures, the edge is either machined or handsanded smooth and flush with the edges of the panel. These potting andcell edge-sealing adhesives are based on epoxy, bismaleimide, andpolyimide chemistries similar to those in prepreg resins. Fillers suchas milled fibers or microballoons as well as flame retardants are alsocommonly used ingredients. The amount of aramid fiber micropulp in apaste or syntactic formulation is generally from 0.05 to 10.0, morepreferably from 0.05 to 6.0 and most preferably from 0.05 to 3.0 weightpercent of the fully formulated composition. The pulp has the sameproperties as that used in the prepreg resin.

EXAMPLES

In the examples, all parts and percentages are by weight and degrees incentigrade unless otherwise set forth:

Example 1

-   Resin can be prepared having the following formulation:-   26.1 percent MY-0510 (N,N-Diglycidyl-4-glycidyloxyaniline)-   24.2 percent GY285 (bis-F epoxy)-   15.3 percent 3,3′-Diaminodiphenylsulfone-   1.3 percent Dicyandiamide-   13.1 percent micronized Polyethersulfone (PES)-   17.0 percent densified Polyethersulfone (PES)-   3.0 percent of aramid fiber micropulp.

The densified PES is made from PES 5003P that is available from SumitomoChemical Co. Ltd. (Osaka, Japan). The PES is densified in accordancewith U.S. Pat. No. 4,945,154. The densified PES has an average particlesize of 1025 micrometers with no more than 13 weight percent smallerthan 5 micrometers and no more than 4 weight percent greater than 40micrometers.

24.2% of GY285 and 6.0% of MY0510 are mixed in a resin kettle andheated, with stirring, to 65 degrees. Once this temperature is attained,13.1% micronized PES 5003P is added to the resin kettle. The mixture isthen heated to 128 degrees and held at this temperature for 75 minutes.At the end of 75 minutes, heating is removed and 20.1% of MY0510 isadded to the kettle. Stirring is continued as the mixture cools to 65degrees. 15.3% of 3,3-DDS is added and mixed for 15 minutes. 1.3% ofdicyandiamide is then added and the mixture stirred for 5 minutes at 65degrees. Finally, 17.0% of densified PES and 3.0% of aramid fiber pulpis added and stirred in for 15 minutes at 65 degrees.

Honeycomb sandwich panels are prepared by first forming a prepreg of 193grams per square meter (gsm) plain weave 3K carbon fiber fabric and 138gsm of resin. The prepreg is formed as follows:

-   -   The resin is coated on release paper by a reverse roll process        at about 79 degrees to form a film containing 69 gsm of resin.        Two resin films are impregnated into the carbon fiber fabric.        The prepreg is applied to ½ inch (1.27 cm) thick, ⅛ inch        (0.31 cm) cell size Nomex® honeycomb type HRH®10 from Hexcel        under vacuum at 22 inches (56 cm) Hg and cured for 2 hours at        177 degrees with a pressure of 45 psi, venting at 20 psi and        ramp cooling at a rate of 2 degrees per minute.

Comparative Example 1

A resin can be prepared having the following formulation:

-   27.0 weight percent MY-0510 (N,N-Diglycidyl-4-glycidyloxyaniline)-   24.9 weight percent GY285 (bis-F epoxy)-   15.8 weight percent 3,3′-Diaminodiphenylsulfone-   1.3 weight percent Dicyandiamide-   13.5 weight percent micronized Polyethersulfone (PES)-   17.5 weight percent densified Polyethersulfone (PES)

The resin mixing procedure is as in Example 1, except that no aramidmicropulp is added. The steps of making prepreg and sandwich panels arealso as in Example 1.

Test coupons from Example 1 and Comparative Example 1 can be subjectedto fluid exposure in a test chamber having controlled temperature andrelative humidity. The samples are weighed before and after exposurewith the weight difference representing the amount of fluid absorbed.Panels containing the fluid barrier prepreg facesheets will have lessfluid ingression. Coupons from a monolithic structure can be testedaccording to ASTM 5229, while for honeycomb structures, a suitable testprocedure is described in “Moisture Ingression in Honeycomb CoreSandwich Panels” by Cise et. al. in the Journal of Materials Engineeringand Performance, Volume 6(6), 732, December 1997.

1. An uncured prepreg composite material for a composite structure, said prepreg comprising: (a) at least one fiber layer; (b) a resin which has been combined with said fiber layer to form a prepreg, said resin comprising a thermoset resin, at least one curing agent and from 0.05 to 10.0 weight percent of a fibrous micropulp, said micropulp fiber having a tenacity of at least 3 grams per denier and a volume average length of from 0.01 to 100 micrometers.
 2. A cured composite material structure compromising the prepreg of claim
 1. 3. A composite material structure of claim 2 further comprising a honeycomb core.
 4. An uncured film adhesive for a composite structure comprising a thermoset resin, at least one curing agent and from 0.05 to 10.0 weight percent of a fibrous micropulp, said micropulp fiber having a tenacity of at least 3 grams per denier and a volume average length of from 0.01 to 100 micrometers.
 5. A cured composite material structure compromising the film adhesive of claim
 4. 6. An uncured liquid or paste resin for a composite structure comprising a thermoset resin, at least one curing agent and from 0.05 to 10.0 weight percent of a fibrous micropulp, said micropulp fiber having a tenacity of at least 3 grams per denier and a volume average length of from 0.01 to 100 micrometers.
 7. A cured composite material structure compromising the liquid or paste resin of claim
 6. 8. A method for making a prepreg comprising the step of combining a prepreg resin with a fiber layer wherein said prepreg resin comprises a thermoset resin, at least one curing agent and from 0.05 to 10.0 weight percent of a fibrous micropulp, said micropulp fiber having a tenacity of at least 3 grams per denier and a volume average length of from 0.01 to 100 micrometers.
 9. A method for making a honeycomb sandwich panel comprising the steps of attaching prepreg according to claim 1 to opposite faces of a honeycomb and curing said prepreg to form said honeycomb sandwich panel. 